LEDs are becoming increasingly popular as a lighting solution. LEDs enable flexible lighting options which make use of the dimming capability of LEDs. However, in order to be compatible with existing dimmable systems, complicated and expensive LED drivers are required.
A basic LED driver typically generates either a constant voltage to be applied to the LED or a constant current. The lowest cost LED drivers do not provide a dimming capability.
One common type of LED driver is a so-called window driver. This driver has a fixed output current, but it can deliver this current while adjusting the output voltage over a relatively large range of values, all within its so-called operating window.
Various possible LED driver solutions have been considered by the inventors, to provide dimming (or other intelligent functions) but making use of a basic LED driver.
One solution is to provide dimming functionality on the LED circuit board, by shunt switching some of the current provided by a constant current driver. The shunt switching involves short-circuiting the LED string with a required duty cycle. Indeed, this short circuit function can be implemented as part of a DC-DC converter, for example by short circuiting a buck converter output. One problem with this approach is that there is typically a capacitor in parallel with the driver output, thus in parallel to the DC-DC converter diodes. Hence the driver acts as a current source with a parallel capacitance. Periodically short circuiting this capacitance can lead to unacceptable energy losses and prohibitively large current peaks.
Another solution is to provide dimming functionality on the LED circuit board using a series switch. This involves making the output load open circuit with a required duty cycle. This can in theory be applied to a constant voltage driver. However, in the case of a constant current driver, the driver may not accept this open circuit. Theoretically, an ideal current source would attempt to compensate for an open circuit by raising the output voltage to infinity. A practical driver would reach a limit voltage and then tolerate a lower current flow. However, a driver may see this as a fault condition. In this case, an over-voltage protection system will switch off the complete supply for a certain period of time.
The capacitor across the driver output is again an issue. During periods that the LED string is disconnected by the series switch, this capacitor will charge up to a higher voltage. When the series switch is closed, the extra charge on the capacitor will lead to a high peak current. This results in an increase of the LED current during on periods. Thus, the total power consumed by the LEDs may not change at all, hence not resulting dimming of the LEDs.
There is therefore a need for a low cost dimming solution which avoids the need for an expensive driver such as a DALI driver (DALI being the acronym for Digital Addressable Lighting Interface), and which can for example be connected to a window driver.